Diurnal and Seasonal Variability of Gas–Particle Partitioning of Secondary Organic Aerosols in the Tropical Atmosphere

Abstract

Gas-particle partitioning is a critical process in the formation and development of secondary organic aerosols, yet its behavior in the atmosphere remains uncertain. The present study examines the diurnal and seasonal characteristics of gas-particle distribution of fine aerosols (PM₁; particulate matter less than 1 μm in aerodynamic diameter) at a tropical coastal station (Thumba; 8.5° N, 77° E, ∼4 m above sea level) in southern peninsular India. Simultaneous measurements of gases and aerosols were collected on a day and night basis using an in-house-developed two-stage low-volume air sampler from January to April 2022 (covering winter and premonsoon). Results show that the chemical composition was characterized by a high abundance of organic carbon (OC) in both phases. While sulfate (SO₄^2–) was found predominantly in the particle phase, nitrate (NO₃^–) and methanesulfonate (CH₃SO₃^–) are prevalent in the gas phase. Oxalate (C₂O₄^2–) was found in both the gas and particle phases. The gas-particle partitioning coefficient (K_p^f) of C₂O₄^2– was higher by 2 orders of magnitude compared to methanesulfonate (CH₃SO₃^–), indicating that C₂O₄^2– has a high tendency to partition into the particle phase. The K_p^f_OC was higher at night than during the day in both seasons. Furthermore, the average daytime values were three times higher in winter than in the premonsoon period. These findings indicate that the gas-particle partitioning mechanism is more prominent at lower air temperatures and higher relative humidity, especially during winter. This inference was further supported by a substantial linear (inverse) relationship between nighttime (daytime) relative humidity (temperature) and K_p^f_OC. A high abundance of gas-phase OC in premonsoon indicates significant volatilisation of particle OC at elevated temperatures.